Cavity filters are used in a variety of applications, for example in nodes of mobile communications networks, specially in radio base stations. Such filters are tuned before put into operation, and a common approach to do so is to adjust a tuning conductor provided at each cavity, for example a centre conductor in the middle of the respective cavity, and/or a lid conductor between two respective cavities. Tuning of such a filter is accomplished by small individual movements of the tuning conductors, thereby changing the magnetic field in the filter.
In known solutions, the tuning conductors are provided in the form of male screws, inserted into female threads at the cavities of the filter. Such solutions include providing the screw with a locking-nut, or a self-locking screw. In the latter case, the screw is provided in the form of a male-threaded part in plastic material, inside of which part, two metal elements are provided. The plastic material provides an isolation between the metal elements and the chassis or lid of the filter. The screws are moved so as to tune the filter, by turning them in the respective female threads of the filter. Thereby, a suitable tool, such as a screwdriver, is used.
A problem with such solutions is that they require high tolerances in the production of the filter. One circumstance that makes it difficult to reach these tolerance requirements, is that silver plating, done on the chassis of the filter during manufacturing, tends to vary and build sharp edges on the female threads. In the case of the self-locking screw, the tolerance requirements are particularly high, since the screw and the corresponding the female thread, in order to achieve a spring action or friction for locking the screw, present an increased diameter at a respective mid-portion thereof.